Because why not? MQTT Light isn’t meant to support some specific hardware device, so you don’t know what capabilities the device is going to have. Why not support as much as possible?
I believe that the SUPPORTED_FEATURES attribute could be dynamically configured based on which topics have been defined in the configuration. In this way the frontend would display controls appropriate to a given instance of an MQTT light. I have not tried this though, so I’m not positive that it would work.
In addition, it would be nice if Light could support passing any arbitrary string as an Effect, rather than a few from a preset list. That way MQTT devices could implement any animations or effects and they could be invoked.
I’m willing to take this on myself if there is interest and no objections. I’m also willing to post the corresponding Arduino sketches for an MQTT light with all of the features I want to add.